Charge-transporting thin films made of organic compounds are used as light-emitting layers and charge-injecting layers in organic electroluminescent (EL) devices. In particular, a hole-injecting layer is responsible for transferring charge between an anode and a hole-transporting layer or a light-emitting layer, and thus carries out an important function for achieving low-voltage driving and high brightness in organic EL devices.
In the past few years, charge-transporting varnishes composed of a uniform solution of a low-molecular-weight oligoaniline-based material or an oligothiophene-based material dissolved in an organic solvent have been discovered and it has been reported that, by inserting a hole-injecting layer obtained from such a varnish in an organic EL device, an underlying substrate leveling effect and excellent organic EL device properties can be obtained (Patent Documents 1 to 3). Moreover, it has also been reported that, by using a 1,4-benzodioxanesulfonic acid compound as an electron-accepting substance (Patent Document 4), the driving voltage of organic EL devices can be lowered.
Yet, because sulfonic acid compounds generally have a low solubility in organic solvents, there tend to be limitations on the solvent used when preparing an organic solution; that is, it has been necessary to use a high proportion of a highly polar organic solvent which has a high solvating power, such as N,N-dimethylacetamide or N-methylpyrrolidone. Organic solutions containing a high proportion of a highly polar organic solvent sometimes cause damage to parts of inkjet coating devices or to organic structures such as insulating films and barrier membranes formed on substrates. Moreover, because sulfonic acid compounds are highly polar, purification by silica gel column chromatography, liquid/liquid extraction, and salt removal by an operation such as water rinsing are difficult.
At the same time, sulfonic acid ester compounds are known to be materials which have a high solubility in various organic solvents and which generate strong organic acids under external stimulation such as heating or chemical action. The cyclohexyl ester of sulfonic acid has been reported as a specific example of a compound which generates sulfonic acid under heating (Non-Patent Document 1). Notice has also been taken of this sulfonic acid ester compound in terms of the concept of a thermal acid generator (Patent Document 5, Non-Patent Document 2). Yet, particularly with regard to sulfonic acid ester compounds substituted on the electron-deficient aromatic ring of an aromatic disulfonic acid or the like, there has existed a desire for the creation of highly stable sulfonic acid ester compounds that readily decompose under slight heating or via reaction with water, a basic substance or the like.